1. Technical Field
The present invention relates to a vibration damping system adapted for use with a structure. More particularly, it relates to a vibration damping system placed at the top of a structure such as a tower of a suspension bridge, a skyscraper, a steel tower, for attenuating vibrations due to wind loads (aerodynamic forces) or an earthquake.
2. Background Art
At least four varieties of the above-mentioned vibration damping system are currently known.
The first is shown in FIG. 13 of the accompanying drawings, and is disclosed in Japanese Patent Application No. 60-92569. In this construction, there is provided a damping device 58 that comprises a device 50 for detecting the vibration amplitude of the vibrating body 52 (this device will be referred to as "vibration sensor" hereafter), an actuator 54 that applies a damping force on the vibrating body 52 corresponding to the vibration amplitude the vibration sensor 50 has detected, and an added mass 56 that balances the damping force as it acts on the vibrating body 52. A stopper unit 60 breaks up excessive motion of the added mass 56, and a damper unit 62 absorbs the impact the added mass 56 exerts on the stopper unit 60.
The second is shown in FIG. 14 of the accompanying drawings and is disclosed in Japanese Patent Application No. 60-92570. This application discloses a system comprising an added mass actuator 66 that is mounted on the structure 64, a vibration sensor 68 that detects the vibration of the structure 64, a controller 70 that outputs control signals on the basis of the signal that the vibration sensor 68 generates, and an added mass actuator 66 that operates under control of the controller 70 so as to damp out the vibration of structure 64. The electric power to be supplied to the added mass actuator 66 is controlled by the timer 76 and on the basis of the output of a structural vibration prediction sensor composed of an earthquake sensor 72 that is set on the ground and a logic circuit 74.
The third is shown in FIGS. 15A and 15B of the accompanying drawings and is disclosed in Japanese Patent Application No. 59-97341. The construction in this prior application comprises an added mass 82 which is placed motion-free on the top 80 of the structure 78, an actuator 88 which drives the added mass 82 and which has its stationary arms 84 affixed to the protrusion 86 extending from the top 80, a controller 90 for controlling the actuator 88, a vibration sensor 92 affixed to the structure 78 for detecting the structure vibration, a ground vibration sensor (an earthquake sensor) 96 set on the foundation 94 of the structure 78 for detecting the ground vibration, and a substractor 98 for subtracting the output signals of the earthquake sensor 96 from the output signals of the vibration sensor 92, and obtaining the input signals for the controller 90.
The fourth is shown in FIG. 16 of the accompanying drawings and is disclosed in Japanese Patent Application No. 60-85165. The construction disclosed in this prior application is a system comprising an added mass actuator 102 attached to the structure 100, and a vibration sensor 104 for detecting vibration of the structure 100, wherein the added mass actuator 102 is driven by a controller 106 and a power source 108 on the basis of the signals from the vibration sensor 104. A band pass filter 110 is provided between the vibration sensor 104 and the added mass actuator 102.
Several deficiencies are evident in the above described constructions. For example, the construction of FIG. 13 is structurally complex because of the provision of the stopper unit 60 and related components; the construction of FIG. 14 requires a complicated controller because of the need for setting the earthquake sensor 72 on the ground; the construction of FIGS. 15A and 15B necessarily calls for duplex vibration sensors because of the use of the subtractor 98; and the construction of FIG. 16 has to bear the burden of a superfluous circuitry because of the presence of the band pass filter 110. In none of these disclosures, moreover, is the method of controlling the actuator presented in concrete terms, and further the phase relationship between the vibration of the structure and the actual movement of the damping device is left unclear.